High-speed diamond growth using a microwave plasma in pulsed mode

ABSTRACT

Disclosed is a method for manufacturing a diamond film of electronic quality at a high rate using a pulsed microwave plasma. The plasma that has a finite volume is formed near a substrate (in a vacuum chamber) by subjecting a gas containing at least hydrogen and carbon to a pulsed discharge. The pulsed discharge has a succession of low-power states and of high-power states and a peak absorbed power P C , in order to obtain carbon-containing radicals in the plasma. These carbon-containing radicals are deposited on the substrate in order to form a diamond film. Power is injected into the volume of the plasma with a peak power density of at least 100 W/cm 3 , while maintaining the substrate to a substrate temperature of between 700° C. and 1000 ° C.

The present invention relates to a method for manufacturing diamondusing a pulsed microwave plasma.

BACKGROUND OF THE INVENTION

Current methods for manufacturing diamond films bymicrowave-plasma-assisted chemical vapor deposition (MP-CVD) are oflimited effectiveness. The large amounts of energy, which are needed toobtain diamond of electronic quality at reasonable growth rates (about 2μm/h), lead to heating of the walls. This process causes hydrogen atomsin the plasma, which activate the reaction, to recombine and notparticipate in the reaction. It is therefore necessary to install aconstricting device for cooling the walls. In the proceedings of theElectrical Chemical Society (ECS) meeting held in San Francisco in 2001,it was proposed in “Diagnostics and modelling of moderate pressuremicrowave H₂/CH₄ plasmas obtained under pulsed mode” by a number ofco-inventors to use a periodic pulsed discharge with a low duty cycle(the ratio of the time during which energy is emitted to the period ofthe discharge), in order to reduce the wall temperature, which isrelated to the average injected power, and therefore the recombinationof hydrogen taking place thereon. Using such a pulsed discharge makes itpossible to maintain a high temperature of the plasma, which is relatedto the power injected during the pulse, and therefore to obtain a higherconcentration of hydrogen atoms in the plasma. Thus, a diamond film maybe deposited at a higher rate for constant consumed power.

BRIEF SUMMARY OF THE INVENTION

The invention relates to a method of this type in which, in a vacuumchamber, a plasma of finite volume is formed near a substrate bysubjecting a gas containing at least hydrogen and carbon to a pulseddischarge, which has a succession of low-power states and high-powerstates, and having a peak absorbed power P_(C), so as to obtain at leastcarbon-containing radicals in the plasma and to deposit the saidcarbon-containing radicals on the substrate in order to form a diamondfilm thereon.

The object of the present invention is to further improve these methods,especially their efficiency.

For this purpose, the invention provides a process for manufacturing adiamond film assisted by a pulsed microwave plasma, which, apart fromthe above mentioned features, is characterized in that power is injectedinto the volume of the plasma with a peak power density of at least 100W/cm³ while maintaining the substrate to a substrate temperature ofbetween 700° C. and 1000° C.

By virtue of these arrangements, it is possible to obtain rapid growthof a diamond film, especially of electronic quality, on the substrate.

In preferred embodiments of the invention, one or more of the followingarrangements may optionally be furthermore employed:

-   -   a plasma having at least one of the following features is        generated near the substrate:        -   the pulsed discharge has a certain peak absorbed power P_(C)            and the ratio of the peak power to the volume of the plasma            is between 100 W/cm³ and 250 W/cm³,        -   the maximum temperature of the plasma is between 3500 K and            5000 K,        -   the temperature of the plasma in a boundary region of the            plasma located less than 1 cm from the surface of the            substrate is between 1500 K and 3000 K and        -   the plasma contains hydrogen atoms having a maximum            concentration in the plasma of between 1.7×10¹⁶ and 5×10¹⁷            cm⁻³;    -   said gas contains carbon and hydrogen in a carbon/hydrogen molar        ratio of between 1% and 12%;    -   said gas contains at least one hydrocarbon and a plasma having a        concentration of the carbon-containing radicals of between        2×10¹⁴ cm⁻³ and 1×10¹⁵ cm⁻³ is generated;    -   a pulsed discharge is produced, in which the ratio of the        duration of the high-power state to the duration of the        low-power state is between 1/9 and 1;    -   at least one of the following parameters is estimated:        -   a substrate temperature,        -   a temperature of the plasma,        -   a temperature of the plasma in said boundary region, located            less than 1 cm from the surface of the substrate,        -   a concentration of atomic hydrogen in the plasma,        -   a concentration of carbon-containing radicals in the plasma,        -   a concentration of carbon-containing radicals in said            boundary region close to the plasma,        -   a pressure of the plasma and        -   a power density of the plasma, and the power emitted as a            function of time is adapted according to at least one of            these parameters;    -   the plasma is contained in a cavity with at least one of the        following properties:        -   the pulsed discharge has a peak power of at least 5 kW at            2.45 GHz,        -   the pressure of the plasma is between 100 mbar and 350 mbar            and        -   the gas containing hydrogen and carbon is emitted with a            ratio of the flow rate to the volume of the plasma of            between 0.75 and 7.5 sccm/cm³;    -   the plasma is contained in a cavity with at least one of the        following properties:        -   the pulsed discharge has a peak power of at least 10 kW at            915 MHz,        -   the pressure of the plasma is between 100 mbar and 350 mbar            and        -   the gas containing hydrogen and carbon is emitted with a            ratio of the flow rate to the volume of the plasma of            between 0.75 and 7.5 sccm/cm³.

Other aspects, objects and advantages of the invention will becomeapparent on reading the description of one of its embodiments which isgiven as a non-limiting example.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will also be more clearly understood from the drawings, inwhich:

FIG. 1 shows one embodiment of the method according to the invention;and

FIGS. 2 a and 2 b are graphs showing a pulsed discharge according to theinvention.

In the various figures, the same references denote identical or similarelements.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an example of how to implement the method according to theinvention using a vacuum chamber 1 containing a support 2 placed on itsbase 3. This vacuum chamber is placed in a Faraday cage 13 acting as acavity or the vacuum chamber itself acts as a cavity. Also, in thevacuum chamber is a single injection nozzle 4 or a plurality ofinjection nozzles. The injection nozzle emits, into the vacuum chamber,gases comprising: a) a source of molecular hydrogen, such as dihydrogen(H₂): and b) a source of carbon, such as a hydrocarbon like methane(CH₄), carbon dioxide (CO₂) or like.

Controlled amounts of argon (Ar) or of dopants and impurities, such asboron (B), sulphur (S), phosphorus (P) or other dopants, may furthermorebe emitted by the injection nozzle 4.

Positioned on the support 2 is a substrate 5, which for example may be asingle-crystal or polycrystalline, natural or synthetic, diamondsubstrate, or even a non-diamond substrate, such as a silicon substrate,whether biased or not, an SiC substrate or an iridium or platinumsubstrate for example.

The gases emitted by the injection nozzle 4 expand into the vacuumchamber and are exposed to a discharge generated by a microwavegenerator 6 such as a GE 60KEDC SAIREM microwave generator operating at2.45 GHz or a microwave generator operating at 915 MHz, the microwavesbeing guided by a waveguide 14. This discharge is coupled to the cavity13 in such a way that the gases form, around the substrate 5, a plasma 7comprising, apart from the molecules of the gases:

-   -   hydrogen atoms H and    -   carbon-containing radicals, for example those in the form of        CH₃, and in general in the form of C_(x)H_(y) or the like.

The plasma 7 may adopt an almost hemispherical shape, for example with adiameter of between 5 cm and 10 cm or other, about the substrate 5. Thecarbon atoms contained in the plasma 7 are deposited on the substrate 5and form a diamond film 8.

The substrate 5 and the diamond film 8 are heated by the surroundingplasma 7 up to a substrate temperature T_(S) of around 700° C. to 1000°C. Furthermore, the temperature of the substrate and of the film may beregulated by a regulating device (not shown) suitable for heating and/orcooling the substrate, this device being contained for example in thesupport 3. This makes it possible, during implementation of the method,to decouple the injected power parameters from the substrate temperatureparameters.

The power generated by the microwave generator 6 is illustrated in FIG.2 a. This power is periodic with time and has, over a period T:

-   -   a peak power P_(C) for a heating time T_(on) and then    -   a low power, relative to the high power, which might be almost        zero, for a standby time T_(off).

The signal is not necessarily strictly periodic during the method, andthe durations of the heating and standby times T_(on) and T_(off) mayvary, for example depending on the conditions measured in the plasma.

Likewise, the emitted power is not necessarily a square wave. For anyperiodic signal, it is possible, over a period, to calculate the meanP_(m) of the emitted power. The emitted power greater than the meanpower defines the heating time T_(on) and is called hereafter the “highpower”. The high power has a maximum instantaneous value called the“peak power” P_(C). The emitted power less than the mean power definesthe standby time T_(off) and is called hereafter the “low power”. Thetimes T_(on) and T_(off) are optionally fractionated over a period.

Within the context of the invention, the peak power P_(C) may have avalue of between 5 kW and 60 kW.

The duty cycle of the microwave generator 6, equal to the ratio of theheating time T_(on) to the period T=T_(on)+T_(off), is between 10% and50%. Thus, the ratio of the time when high power is emitted to the timewhen low power is emitted may be between 1/9 and 1.

Apart from in a transient regime at the start of the heating timeT_(on), having a duration much less than T_(on), during which the plasmavolume varies, principally increasing, the plasma has during the heatingtime T_(on) a generally constant volume directly related to the pressureof the plasma, which in practice is between approximately 100 mbar and350 mbar, and to the microwave frequency of the microwave generatorused. The rest of the description ignores the transient state occurringat the start of the heating time, taking into account only the “steadystate” of the plasma that occurs thereafter.

Such a periodic pulsed discharge is used to obtain a pulsed plasma whosetemperature remains high, thereby guaranteeing high concentrations ofhydrogen atoms H and carbon-containing radicals and therefore a highdeposition rate, while maintaining a low temperature of the walls 13 ofthe vacuum chamber 1. With such an absorbed power, the temperature ofthe plasma 7 rises up to a maximum value of between 3500 K and 5000 K.Consequently, and depending on the volume of the plasma 7, the powerdensity corresponding to the peak power injected into the plasma isbetween 100 W/cm³ and 250 W/cm³. This power density is calculated as theratio of the peak power P_(C) to the volume of the plasma 7, which maybe measured by specific measurement means such as, for example, opticalspectroscopy, or by a high-speed optical camera of the “Flash Cam” type,for example in the visible range, or by other means. The gas temperaturein a boundary region of the plasma, located less than 1 cm from thesurface of the substrate, between the substrate and the generator, mayalso be between 1500 K and 3000 K.

These conditions greatly favor the disruption of the molecular hydrogenH₂ emitted by the injection nozzle 4 and the formation ofcarbon-containing radicals. A concentration between 1.7×10¹⁶ cm⁻³ and5×10¹⁷ cm⁻³ of atomic hydrogen in the plasma may be measured. Suchatomic hydrogen concentrations make it possible to increase the reactionrate for depositing the carbon-containing radicals contained in theplasma in the form of a diamond to a high reaction rate, whileguaranteeing the electronic quality of the diamond film produced. Theseconditions thus advantageously allow the concentration ofcarbon-containing radicals in the plasma to be increased so that thelatter may contain between 2×10¹⁴ cm⁻³ and 1×10¹⁵ cm⁻³ CH₃ radicals.Since the incorporation of carbon atoms into the diamond film 8 beingformed is substantial, the molecular methane may be emitted by theinjection nozzle 4 with a molar ratio of possibly up to 12% (withrespect to molecular hydrogen, H₂).

In the considered embodiment, the volume of the plasma is kept overallconstant at 65 cm³ by a flow via the injection nozzle 4 with a flow rateof between 50 sccm and 500 sccm, which corresponds to a ratio of theflow rate to the volume of plasma of between 0.75 and 7.5 sccm/cm³ forexample. Of course, it is unnecessary for the plasma to maintain aconstant volume during the method, nor indeed does this volume have tobe around 65 cm³. The volume of the plasma may be modified by regulatingits pressure within the 100 mbar-350 mbar range. Furthermore, the volumeof the plasma may also be increased or reduced by using a microwavegenerator at a lower or higher microwave frequency respectively.

As explained above, using a controlled pulsed discharge allows thecharacteristics of the plasma to be increased, in particular the atomichydrogen and carbon-containing radical concentrations therein, since thetemperature of the plasma can be increased while the wall temperature,directly related to the mean power of the discharge, remains low. Thesignificant parameters governing the growth of the diamond film are thusdirectly related to the peak power.

Thus, by reducing the heating time T_(on) for a given period, and for agiven mean power, the peak power P_(C) may be increased up to maximumvalues ranging from 6 kW to 60 kW, depending on the generator used. Thereaction rate is related to the concentration of atomic hydrogen and ofcarbon-containing radicals in the plasma 7 and by the temperature of thesubstrate T_(S). On the other hand, the mean power of the dischargecycle must remain low so as to avoid an excessively high temperature ofthe walls 13 of the vacuum chamber 1, which leads, for a constant periodT of the discharge cycle, to reducing the heating time T_(on) andincreasing the standby time T_(off). During that part of the dischargecycle between T_(on) and T, a low, even zero, microwave power isinjected into the plasma 7 so that the radicals in this plasmarecombine. Thus, the concentration of atomic hydrogen H in the plasma 7decreases during this time interval and the atoms recombine intohydrogen molecules H₂, which again will have to be disrupted during thenext discharge, thereby reducing the efficiency of the process. Duringthe standby time T_(off), the atomic hydrogen concentration decreaseswith time, characterized by a lifetime T_(V) of the hydrogen atoms inthe plasma that depends on the temperature and pressure conditions ofthe plasma. It is desirable to try to limit the process of hydrogenatoms recombining during the standby time T_(off) so as to have todisrupt the minimum amount of hydrogen molecules H₂ during the nextheating time T_(on).

The invention makes it possible to obtain a pulsed microwave plasmausing an energy source 6 delivering a periodic discharge with time, thestandby time T_(off) of which is strictly shorter than the lifetimeT_(V) of the hydrogen atoms in the plasma 7.

The lifetime T_(V) of the atomic hydrogen H in the plasma 7 may bedetermined, for example, by a known plasma induced fluorescence (PIF)technique. As shown in FIG. 2 b, PIF technique consists of generatingthe first power peak that has the duration T_(on) and the peak powerP_(C), and the second power peak that occurs at a defined time T₀ takenbetween T_(on) and T. The second peak is of short duration, for exampleabout 1/10 of T_(on). The second peak, by direct collision with anelectron, excites the hydrogen atoms H still present in the plasma 7 attime T₀. This excitation is measured and compared with the excitationcaused by the first peak of the discharge, thereby making it possible todetermine the concentration of hydrogen atoms H remaining in the plasma7 at time T₀. Determining the concentration of hydrogen atoms remainingin the plasma thus allows to determine the hydrogen atom lifetime underthe given conditions of the plasma. Optionally, this information may betransmitted to the microwave generator 6 which adapts the parameters ofthe discharge accordingly. Other known techniques, such as laser-inducedstimulated emission (LISE) or two photon laser-induced fluorescence maybe used in this context.

Measures may also be taken to ensure that, during the standby timeT_(off), a residual power P_(R) of about 10% of the peak power P_(C) isinjected into the plasma so that the microwave generator 6 remainsactive and can deliver more rapidly, at the start of each new dischargecycle period, a high peak power P_(C).

1. A method for manufacturing a diamond film comprising: forming aplasma of finite volume near a substrate by subjecting a gas containingat least hydrogen and carbon in a vacuum chamber to periodic pulseddischarges using a pulsed microwave plasma by applying only a repeatedsuccession of a low-power state and a high-power state, in which theratio of the duration of the high-power state to the duration of thelow-power state is between 1/9 and 1, and having a peak absorbed powerP_(C) so as to obtain at least carbon-containing radicals in the plasma,and depositing the said carbon-containing radicals on the substrate inorder to form a diamond film thereon; wherein the power being injectedinto the volume of the plasma with a peak power density of at least 100W/cm³ while maintaining the substrate to a substrate temperature ofbetween 700° C. and 1000° C. and also wherein the pressure of the plasmais maintained between 100 mbar and 350 mbar.
 2. The method according toclaim 1, in which a plasma having at least one of the following featuresis generated near the substrate: the peak power density of the plasma isbetween 100 W/cm³ and 250 W/cm³, the maximum temperature of the plasmais between 3500 K and 5000 K, the temperature of the plasma in aboundary region of the plasma located less than 1 cm from the surface ofthe substrate is between 1500 K and 3000 K and the plasma containshydrogen atoms having a maximum concentration in the plasma of between1.7×10¹⁶ and 5×10¹⁷ cm⁻³.
 3. The method according to claim 1 or claim 2,in which said gas contains carbon and hydrogen in a carbon/hydrogenmolar ratio of between 1% and 12%.
 4. The method according to claim 1,in which said gas contains at least one hydro-carbon, and a plasmahaving a concentration of the carbon-containing radicals of between2×10¹⁴ cm⁻³ and 1×10¹⁵ cm⁻³ is generated.
 5. The method according toclaim 1, in which at least one of the following parameters is estimated:a substrate temperature, a temperature of the plasma, a temperature ofthe plasma in said boundary region, located less than 1 cm from thesurface of the substrate, a concentration of atomic hydrogen in theplasma, a concentration of carbon-containing radicals in the plasma, aconcentration of carbon-containing radicals in said boundary regionclose to the plasma, a pressure of the plasma and a power density of theplasma, and the power emitted as a function of time is adapted accordingto at least one of these parameters.
 6. The method according to claim 1,in which the plasma is contained in a cavity with at least one of thefollowing properties: the periodic pulsed discharges have a peak powerof at least 5 kW at 2.45 GHz and the gas containing hydrogen and carbonis emitted with a ration of the flow rate to the volume of plasma ofbetween 0.75 and 7.5 sccm/cm³.
 7. The method according to claim 1, inwhich the plasma is contained in a cavity with at least one of thefollowing properties: the periodic pulsed discharges have a peak powerof at least 10 kW at 915 MHz and the gas containing hydrogen and carbonis emitted with a ratio of the flow rate to the volume of the plasma ofbetween 0.75 and 7.5 sccm/cm³.